A capacitive touch sensor, referred to simply as a touch sensor in the following, may detect the presence and location of a touch or the proximity of an object (such as a user's finger or a stylus) on a surface. Touch sensors are often combined with a display to produce a touch screen. For a touch screen, the most common display technologies currently are thin film transistor (TFT) liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays. In other devices, the touch sensors are not combined with a display, e.g. a touch pad of a laptop computer. A touch screen enables a user to interact directly with what is displayed on the screen through a graphical user interface (GUI), rather than indirectly with a mouse or touch pad. A touch sensor may be attached to or provided as part of a mobile phone, tablet or laptop computer, for example.
Touch sensors may be classified into grid and matrix types. In a matrix type, an array of electrodes is arranged on the surface which are electrically isolated from each other, so that each electrode in the array provides its own touch signal. A matrix type touch sensor is therefore naturally suited to situations in which an array of touch-sensitive buttons is needed, such as in a control interface, data entry interface or calculator. In a grid type, there are two groups of parallel electrodes, usually referred to as X and Y electrodes, since they are typically arranged orthogonal to each other. A number of nodes are defined by the crossing points of pairs of X and Y electrodes (as viewed in plan view), where the number of nodes is the product of the number of X electrodes and Y electrodes. A grid type touch sensor is the type typically used for touch screens on mobile phones, drawing tablets and so forth. In earlier designs, the X and Y electrodes are arranged either side of a dielectric layer, so they are vertically offset from each other by the thickness of the dielectric layer, vertical meaning orthogonal to the plane of the layers. In more recent designs, to reduce overall thickness, the X and Y electrodes are deposited on the same side of a dielectric layer, i.e. in a single layer, with thin films of dielectric material being locally deposited at the cross-overs to avoid shorting between the X and Y electrodes. A single electrode layer design of this kind is disclosed in US 2010/156810 A1, the entire contents of which are incorporated herein by reference.
Touch sensors may also be classified into self capacitance and mutual capacitance types.
In a self capacitance measurement, the capacitance being measured is between an electrode under a dielectric touch panel and the touching finger, stylus etc., or more precisely the effect that the touch's increase in capacitance with the electrode has on charging a measurement capacitor that forms part of the touch IC's measurement circuit. The finger and the electrode can thus be thought of as acting as the plates of a capacitor with the touch panel being the dielectric.
In a mutual capacitance measurement, adjacent pairs of electrodes are arranged under the touch panel, and form the nominal plates of the capacitor. A touching body acts to modify the capacitance associated with the electrode pair by replacing what was the ambient environment, i.e. in most cases air, but possibly water or some other gas or liquid, with the touching object, which may be effectively a dielectric material (e.g. a dry finger, or a plastics stylus) or in some cases could be conductive (e.g. a wet finger, or a metal stylus). One of the pair of electrodes is driven with a drive signal, e.g. with a burst of pulses, and the other electrode of the pair senses the drive signal. The effect of the touch is to attenuate or amplify the drive signal received at the sense electrode, i.e. affects the amount of charge collected at the sense electrode. Changes in the mutual capacitance between a drive electrode and a sense electrode provide the measurement signal. It is noted that in a mutual capacitance grid sensor, there is a convention to label drive electrodes as the X electrodes and sense electrodes as the Y electrodes, although this choice is arbitrary. A perhaps clearer labelling that is often used is to label the drive electrodes as “Tx” for transmission and the sense electrodes as “Rx” for receiver in analogy to telecoms notation, although this labelling is of course specific to mutual capacitance measurements.
Current industry standard touch screens for mobile phones rely on operating the same touch sensor to make both self capacitance and mutual capacitance measurements, since acquiring both is beneficial to gaining additional information about the touch which can be used in post-processing to increase the reliability of interpretation. For example, mutual capacitance measurement have high noise immunity, whereas self capacitance measurements are easier to interpret and give a direct measure of moisture presence.
FIG. 1A is a schematic cross-section through a touch panel in a plane perpendicular to the plane of the stack showing a mutual capacitance measurement involving an individual pair of X (drive) and Y (sense) electrodes: X, Y. Electric field lines are shown schematically with the arrow-headed, curved lines.
FIG. 1B is a schematic cross-section through the same touch panel as FIG. 1A in the same plane showing a self capacitance measurement involving the same pair of X and Y electrodes: X, Y. Electric field lines are shown schematically with the arrow-headed lines.
In touch screen design, there is a continuing trend towards making the display and sensor stack thinner so the whole phone, tablet etc. can be as thin as possible. Generally, a thinner stack means that the display layers, in particular the display drive electrodes are brought closer to the touch sensor layers, in particular the touch sensor electrodes. There is also a desire to make the touch panel thinner, although this is motivated by a desire to reduce cost (since the touch panel material is expensive) or to provide for flexibility of the display stack.
A side effect of bringing the display ever closer to the touch sensor electrodes, is that there is ever larger self capacitance between the touch sensor electrodes and the display electrodes. An unwanted consequence of the proximity of the display electrodes to the touch sensor electrodes is signal inversion of poorly grounded touches in mutual capacitance measurements. When a touch is received from a finger or other touching object which is not well grounded to the system ground, this is referred to as a floating touch, as opposed to a grounded touch. A floating touch is defined as one having a low self capacitance to the system ground, and a grounded touch is defined as one having a high self capacitance to the system ground. In a mobile phone or other handheld device, the system ground may be constituted by the device chassis or housing, and/or by the display electrodes. If a user is holding the device, then the user, and hence his or her touches, can be expected to be well grounded. However, if the device is not being held, e.g. lying on a well insulated object such as a wooden desk top or a fabric car seat, then the device itself is electrically isolated, so grounding of a touch will be dependent on the touch being able to find a ground to the device itself.
A floating touch can cause an undesired increase in mutual capacitance between drive and sense electrodes in a touch sensor layer, instead of the usual, desired decrease. The sign of the touch signal is thus inverted. A concrete example has a touch sensor electrode configuration with co-planar X and Y electrodes arranged 0.12 mm above the display electrodes. The X and Y electrodes are both the same size at 6×3 mm and arranged adjacent to each other along their 6 mm sides separated by a gap of 0.135 mm. The touch panel thickness is 0.1 mm. All dielectric permittivities are taken to be 3.5 or 4.0 for the various dielectric layers including the touch panel. Other parameters are set to typical representative values. With this example, we find that when a grounded touch of 4 mm diameter touches the touch surface, then the mutual capacitance reduces to approximately half the no-touch value. More specifically, when there is a no-touch mutual capacitance of approximately 200 fF, which reduces to about 100 fF with the grounded touch. On the other hand, with a floating touch of the same size, the mutual capacitance increases to approximately 600 fF, i.e. increases to 3 times the no-touch value. In any given touch sensor and display combination, i.e. touch screen stack, simulations or testing can be used to determine, based on touch size and grounding state of the touch, whether the mutual capacitance change will be positive or negative. Because of its predictability and reproducibility, this effect can be dealt with in post-processing by the touch-sensor controller chip. A self capacitance measurement can be used to detect how well a touch is grounded, and this information can be used as an aid when analysing the mutual capacitance measurement data. However, in thin sensor and display stacks, the ability to carry out self capacitance measurements is compromised, since the proximity of the display electrodes to the touch sensor electrodes can lead to the capacitance between touching object and touch sensor electrode becoming much smaller than that between the display electrodes and the touch sensor electrodes. Consequently, there may be no accurate self capacitance measurements available to use as a post-processing aid to detecting signal inversion in the mutual capacitance measurements.